As is known, all electronic devices that include integrated circuits require at least one DC voltage supply and typically requires multiple DC voltage supplies. A DC voltage supply may be generated from an AC voltage source (e.g., 110 volts AC) or from another DC voltage supply (e.g., a battery). To generate a DC voltage supply from an AC voltage, the AC voltage is processed in a controlled manner. For example, a switch-mode power supply will rectify the AC voltage to produce a DC bridge voltage. Using one of a plurality of switch mode converter topologies (e.g., full bridge, half bridge, buck, or boost) an inductor is charged and discharged at a controlled rate to produce a regulated DC voltage supply.
When only one DC output voltage is needed, a well-regulated power supply system is readily achievable. When multiple DC output voltages, or supplies, are needed, however, design choices must be made to optimize the performance of the multiple output power supply. If power consumption is not a significant issue, but well-regulated multiple DC output is, then linear regulators may be used for the auxiliary outputs and direct regulation of the primary DC output supply. While the linear regulator will accurately produce the three-volt output from a five-volt source, it is inefficient since that for every three watts of output power produced, two watts are consumed.
In an alternate design choice, if power consumption is a critical factor, but regulation of auxiliary supplies, (e.g., the three volts in the preceding example) is not a critical factor, then a multi-tap transformer may be used in place of the inductor. A secondary tap on the transformer produces the auxiliary DC output and a primary tap produces the primary DC output. In this embodiment, only the primary output is regulated. Thus, as the load varies on the primary DC output, the auxiliary DC output will vary by as much as ten percent (10%).
In designs where both power consumption and well-regulated multiple outputs are significant factors, DC to DC converters are used. As is known, a DC to DC converter includes its own inductor and control circuit to regulate a DC output from a DC input. Thus, multiple inductors and multiple control circuits are needed. As with most electrical devices, size and cost are concerns. Thus, having multiple DC to DC converters to produce regulated power supply voltages is prohibitive to reducing size and reducing costs of such devices, especially when at least partially implemented on an integrated circuit.
Recent advances in DC to DC converter design have produced multiple outputs from a single inductor. FIG. 1 illustrates an embodiment of the transistor switching circuitry of a boost converter producing two outputs from a single inductor. As shown, this portion of the boost converter includes three switching transistors (S1, S2 and S3), an inductor (L), a battery (Vbatt) and two capacitors (C1 and C2). The boost converter produces two outputs (Vout1 and Vout2). In operation, a charge signal turns on transistor (S1) such that a current flows through the inductor (L) to charge it, i.e. builds electromagnetic energy within the inductor. When the charge signal is removed, turning transistor S1 off, either load signal 1 or load signal 2 is activated turning on the corresponding transistors S2 or S3. For instance, if load signal 1 is activated, S2 is enabled such that at least some of the energy built-up in inductor L is transferred to capacitor C1 thereby producing output voltage Vout1. Similarly, when load 2 signal is activated, the energy from the inductor is provided to capacitor C2 thereby producing output Vout2.
An issue with the boost converter of FIG. 1 is that unwanted output voltage ripple is generated when switching between the loads (i.e., switching between enabling load 1 signal and load 2 signal). FIG. 2 depicts an illustrative example of the ripple. FIG. 2 illustrates a load select signal, the charge signal, load 1 signal, load 2 signal and the inductor current. The load select signal indicates whether load 1 signal will be activated (i.e., signal is in the high state) or whether load 2 signal (i.e., the signal is in the low state) will be activated. As shown, initially the load select is selecting the 1st output (Vout1) such that load 1 signal will be activated.
When the charge signal is high, transistor S1 is on such that current is flowing through the inductor from the battery to ground via S1. As shown, the current rises during the activation of the charge signal. When the charge signal is disabled and load signal 1 is enabled, at least some of the energy is transferred from the inductor to capacitor C1 via S2. During this time, the inductor current decreases as shown. The ratio between the on-time of the charge signal and the on-time of load 1 signal is dependent on the output voltage Vout1 and the battery voltage Vbatt, in a steady state condition.
The charging and discharging of the inductor and capacitor C1 continues in the manner as previously described while load select signal remains in the high state. When the load select signal transitioned to the low state such that C2 will receive the energy from the inductor, the control circuitry (not shown in FIG. 1) begins to adjust the loop response accordingly. For this example, the control circuitry begins to increase the duty cycle of the charge signal, since Vout2 is greater than Vout1. As shown, at the transition of the load select signal, the charge signal has minimal pulse width change, with respect to the duty cycle for Vout1, due to the relatively slow loop response of the control circuitry with respect to the switching frequency. As such, the charge signal causes the inductor current to increase almost as if it were being charged for the first output Vout1. When the charge signal is disabled and load 2 signal is enabled, the current of the inductor decreases substantially in comparison to the decease for load 1. As the control loop continues to adjust for providing energy to capacitor C2 for Vout2 the charge signal continues to increase in pulse width thereby increasing the “charge” current for the inductor. At some point in time, a steady state condition will be reached for regulating the 2nd output Vout2. However, during the transition time, as illustrated, the inductor current is offset from a DC average. This offset is directly reflected in the output voltage Vout2 as ripple. Such output ripple in some integrated circuit applications is unacceptable.
Therefore, a need exists for a method and apparatus of regulating multiple outputs of a single inductor DC to DC converter with reduced output ripple.